For keeping up with recent environmental problems, various types of materials have been studied. There are three strategies, i.e., (1) reduction in dependency on fossil resources, (2) biodegradability for reduction in environmental load, and (3) improvement in recyclability of materials and easy regeneration of resources. Polylactic acid has been studied continuously over 20 years as a material that is derived from a biomass and has biodegradability. It has started to be used for fibers, films, containers and molding materials, but slow crystallization rate, low strength, low heat resistance, low barrier property, and poorer balance between properties and price in comparison to general purpose petroleum resins have been pointed out as problems. In particular, improvement in properties, namely, heat resistance, solvent resistance and strength, is important and various studies are being made at present.
As a most-widely known technology for improving properties themselves of a resin, there is known a technology of forming a stereocomplex. A stereocomplex is a crystal in which a segment of a poly-L-lactic acid (hereinafter abbreviated as PLLA) and a segment of a poly-D-lactic acid (hereinafter abbreviated as PDLA) are packed in a one-to-one ratio and this can increase the melting point of PLLA alone or PDLA alone by about 50° C. In addition to that, it is known that mechanical properties, solvent resistance, and gas barrier property are improved by stereocomplex formation, and this is researched currently by many companies.
As to a method for producing a stereocomplex, it can be obtained by blending PLLA and PDLA, but there are the following problems with practical production as is apparent from prior art documents.
(1) When using a high molecular weight polylactic acid, which is advantageous in an aspect of mechanical properties, it is difficult to form a stereocomplex efficiently during the course of crystallization, and a large number of homocrystals of PLLA and PDLA, which are species to preferentially crystallize, and a small number of stereocomplex crystals are formed by merely blending. Therefore, originally intended improvement in heat resistance and so on becomes insufficient.
(2) Although it has been known that the efficiency of stereocomplex formation is improved by kneading in a molten state, degradation of a resin caused by heat and fall of the melting point of a stereocomplex PLA due to transesterification between PLLA and PDLA also occur and, therefore, an effect sufficient for the intended goal can not be obtained.
With respect to these points, converting PLLA and PDLA into a block polymer (patent document 1), improving the efficiency of stereocomplex formation by reducing the molecular weight of one polylactic acid resin (patent document 2), forming a stereocomplex efficiently in a state where the molecular weight is low, and then performing solid phase polymerization (patent document 3), copolymerizing another component to one polylactic acid in order to improve the compatibility with another polylactic acid (patent document 4), and performing heat treatment at a specified temperature for forming a stereocomplex (patent document 5) have been proposed for stereocomplex formation. However, these methods are all not on the precondition of using a general purpose polylactic acid but on the precondition of improving a resin itself, and, therefore, there are at present many problems to be solved for industrialization. Moreover, there have not been proposed any solutions for the problem that these methods are accompanied by a transesterification reaction in promoting stereocomplex formation or in melt-molding a resulting resin composition again, resulting in occurrence of fall of a melting point and so on.
Moreover, in the case of subjecting a general polyester resin or the like to processing such as drawing, it is general to heat it to a temperature equal to or higher than its melting point to once melt its crystals, rapidly cool the melt into an amorphous state, and process the amorphous within a temperature region of from the glass transition point to the melting point. By a method in which a stereocomplex polylactic acid is heated once to about 250° C., which is a temperature higher than the melting point (140 to 170° C.) of a polylactic acid resin, thereby melting crystals of the polylactic acid and followed by rapidly cooling the melt and the resulting molded article is subjected to drawing, rupture occurs due to fall of drawability caused by stereocomplex crystals in a step of drawing or the like, and a drawn film was not obtained probably because the stereocomplex crystals in the resin can not be melted completely. Then, it becomes possible to draw a molded article by heating a stereocomplex polylactic acid to a temperature of 280° C. or higher at which crystals of the stereocomplex polylactic acid can melt completely; however, the melting point of the resulting resin becomes a temperature that is far lower than the original melting point of a stereocomplex crystal. This is probably because it has been pointed out that a stereocomplex is difficult to be formed again depending upon the melting state of stereocomplex crystals which depends on the temperature, the melting time and the like at the time of melting (non-patent document 1) and if stereocomplex crystals are melted once, homocrystals of PLLA and PDLA, which are species to preferentially crystallize, are formed earlier. As described above, there is at present no method of industrially reconciling molding processability with efficiency of reformation of a stereocomplex polylactic acid crystal after remelting.
Among the methods described in the above-listed patent documents, a concrete production method is a method in which the preparation of a stereocomplex is carried out by casting film from a solution. However, this method is not suitable for industrial mass production, or there have been obtained only films resulting from pressing at temperatures as low as about 250° C., or the melting point has been lowered or a sufficient stereocomplex has not been obtained even in a product obtained by melting.
The above-described known technologies developed up to date are summarized as follows.
(i) A stereocomplex body is difficult to be formed by only melt-blending PLLA and PDLA. Moreover, a melting point falls due to a transesterification reaction if kneading PLLA and PDLA under melting in order to promote stereocomplex formation. Even if a stereocomplex is formed, the stereocomplex is easily divided into PLLA and PDLA by remelting to each form a stereo single crystal, and a complex is hardly formed again.
(ii) Reformation of a stereocomplex in remelting is promoted by methods such as block polymerization and reduction in the molecular weight of one resin, but the fall of melting point due to a transesterification reaction can not be suppressed.